This means video re-scaling.
1. Publish video 640x480 (4:3)
2. Play video 400x225 (16:9)
If resample_video=true, WCS will rescale video from 640x480 to 400x225 and it will be flattened vertically.
If resample_video=false, WCS will cut video to 400x225, and part of video will be lost.
So, if you set playback width and height, you have to set correct ratio, i.e. 320x240 for 640x480 publishing stream, then resample_video=true will process video properly.
This setting is true, by default.